Coolant Overflow Tanks & Expansion Catch Cans

As an engine warms up, it heats the coolant within. This hot coolant circulates between the radiator and engine. When the hot coolant reaches the radiator, the heat from it transfers to the radiator fins where it is removed by airflow from vehicle movement or by a fan. As the coolant gets hotter and hotter, the molecules expand. This temporary excess coolant needs a place to go. The engine coolant overflow tank, typically made of plastic or metal such as aluminum, allows the expanding coolant a place to move to via coolant expansion tank plumbing; typically a hose between the radiator and tank. When the engine cools down or is turned off, the coolant slowly lowers in temperature and the molecules contract. When this happens, it creates low pressure and suction within the radiator coolant overflow tank hose and draws fluid back into the radiator.

Most overflow tanks have a minimum and maximum level marked on the side to help you confirm the correct amount of fluid. Otherwise, a general rule of thumb is that on a cold engine, the coolant overflow tank filling up until its approximately 1/3 full.

There are some possible causes for boiling coolant in a coolant catch can. The first possibility is a bad radiator cap. The radiator cap (either on the radiator or inline on a hose) has a spring that allows it to release pressure above its designed amount. When this happens, the outlet leading to the overflow tank is opened and coolant is able to push and expand in the hose to it. If the radiator cap does not work properly, pressure drops in the system and the boiling point is lowered, causing coolant overflow tank bubbling. Another possibility is an overheated engine, which causes the coolant to get hotter and reach the boiling point.

There are a couple of ways to perform a diy draining of an overflow tank. One is with a pump to transfer coolant out of the opening from the removal of the coolant overflow tank cap. The other is to remove the hose from the bottom of the tank or at the radiator. In both cases, have a container large enough to collect all of the fluid within the hose and coolant expansion tank.

A JEGS expert is ready to answer your questions and help you find the coolant overflow tank that you require for your specific application.